The present invention relates to substrate processing. More specifically, the present invention relates to apparatus and methods for upgrading a substrate processing system. Some embodiments of the present invention are particularly useful for cleaning a chamber in a substrate processing system. However, other embodiments of the present invention also may be useful for etching or depositing films on a substrate processed in the substrate processing system.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a layer, such as a metal silicide layer like tungsten silicide (WSi.sub.x), on a substrate or wafer. As is well known, such a layer can be deposited by chemical vapor deposition (CVD). In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions take place to form the desired film over the surface of the substrate being processed. In a conventional plasma-enhanced CVD (PECVD) process, a controlled plasma is formed using radio frequency (RF) energy or microwave energy to decompose and/or energize reactive species in reactant gases to produce the desired film.
One problem that arises during such CVD processes is that unwanted deposition occurs in the processing chamber and leads to potentially high maintenance costs. With CVD of a desired film on a wafer, undesired film deposition can occur on any hot surface including the heater or process kit parts of the apparatus, because the reactive gases can diffuse everywhere, even between cracks and around corners, in the processing chamber. During subsequent wafer depositions, this excess growth on the heater and/or other parts of the apparatus will accelerate until a continuous metal silicide film is grown on the heater and/or these other parts. Over time, failure to clean the residue from the CVD apparatus often results in degraded, unreliable processes and defective wafers. When excess deposition starts to interfere with the CVD system's performance, the heater and other process kit parts (such as the shadow ring and gas distribution faceplate) can be removed and replaced to remove unwanted accumulations in the CVD system. Depending on which and how many parts need replacing and the frequency of the replacement, the cost of maintaining the substrate processing system can become very high.
In these CVD processes, a reactive plasma cleaning is regularly performed in situ in the processing chamber to remove the unwanted deposition material from the chamber walls, heater, and other process kit parts of the processing chamber. Commonly performed between deposition steps for every wafer or every n wafers, this cleaning procedure is performed as a standard chamber cleaning operation where the etching gas is used to remove or etch the unwanted deposited material. Common etching techniques include plasma CVD techniques that promote excitation and/or disassociation of the reactant gases by the application of RF energy with capacitively-coupled electrodes to a reaction zone proximate the substrate surface. In these techniques, a plasma of highly reactive species is created that reacts with and etches away the unwanted deposition material from the chamber walls and other areas. However, with some metal CVD processes, etching gases useful for etching unwanted metal are often corrosive and attack the materials which make up the chamber, heater, and process kit parts of the processing chamber. Moreover, use of in situ plasma cleaning also causes ion bombardment of the metallic parts of the CVD apparatus, causing physical damage to the gas distribution manifold and the inside chamber walls. Therefore, in situ cleaning with these etching gases may make it difficult to effectively clean excess CVD film without also eventually damaging the heater and other chamber parts in the cleaning process. Thus, maintaining chamber performance may result in damage to expensive consumable items which need frequent replacement as a result. In addition to such in situ plasma cleaning procedures and occurring far less frequently, a second cleaning procedure (often referred to as a preventive maintenance cleaning) involves opening the processing chamber and physically wiping the entire reactor--including the chamber walls, exhaust and other areas having accumulated residue--with a special cloth and cleaning fluids. Without these frequent cleaning procedures, impurities from the build up in the CVD apparatus can migrate onto the wafer and cause device damage. Thus, properly cleaning CVD apparatus is important for the smooth operation of substrate processing, improved device yield and better product performance.
As an alternative to in situ plasma cleaning, other conventional CVD apparatus have a separate processing chamber connected to a remote microwave plasma system. Because the high breakdown efficiency with a microwave plasma results in a higher etch rate (on the order of about 2 .mu.m/min) than is obtained with a capacitive RF plasma, these remote microwave plasma systems provide radicals from the remote plasma that can more gently, efficiently and adequately clean the residue without ion bombardment.
However, these conventional remote microwave plasma systems often require expensive and fragile equipment for operation. FIG. 5 illustrates an exemplary remote microwave plasma system according to the prior art. In many of these conventional CVD apparatus, the remote microwave plasma system includes a ceramic plasma applicator tube 601, a conventional magnetron 603 (coupled to a power source, not shown) with an antenna 604, isolator (not shown), ultra-violet (UV) lamp 605 with power supply 607, and bulky waveguide system 609 with tuning assembly (not shown). Ceramic applicator tube 601 includes a gas inlet 613 connected to a gas source (not shown) for introduction of a reactive gas into the tube 601, where microwaves passing through the portion of tube disposed within a portion of waveguide 611 radiate the reactive gas, which is ignited by UV lamp 605 to form a plasma in a space 613a. Radicals exit an outlet 615 of ceramic tube 601 that is connected to a downstream processing chamber. Such conventional remote microwave plasma systems produce plasma in the relatively small physical space 613 (for example, about a two-inch lengthwise section of a ceramic applicator tube having about a 1 inch diameter) in the ceramic applicator tube 601, having a total length of about 18-24 inches, which is disposed through a portion of the waveguide 611 in waveguide system 609. The plasma formed in this small space 613a of the ceramic applicator tube 601 by magnetrons using high power supplies has a high plasma density and requires expensive, high power density, direct current (DC) microwave power supplies in order to obtain sufficiently high microwave coupling efficiency. Since the plasma formed in small space 613a has such a high plasma density, the ceramic applicator tube 601 often becomes very hot. Such ceramic applicator tubes, which are subject to cracking and breakage after repeated thermal cycling, can be expensive to replace. Additionally, some of these conventional remote plasma sources may require a UV lamp or a microwave source with very high wattage (on the order of 3 kilowatts (kW)) in order to ignite the plasma.
From the above, it can be seen that it is desirable to have an economic, robust remote microwave plasma system that permits efficient cleaning of a downstream substrate processing apparatus. It is also desirable to provide a remote microwave plasma system that provides more efficient generation of reactive radicals for cleaning the downstream substrate processing apparatus. A relatively inexpensive, yet high quality, remote microwave plasma source that may be a removable addition to or a retrofit of existing substrate processing apparatus, is needed in order to upgrade performance of the apparatus for improved cleaning ability while minimizing costs.